When extremely sensitive detectors are employed, their extreme sensitivity makes them especially sensitive to environmental factors that can produce undesirable electronic noise that can obscure the true signal. Some examples of such environmental factors include interfering electromagnetic waves that are periodic or random (electromagnetic interference or EMI), thermally-induced effects such as, for example, thermally induced current (dark current), circuit and instrumental operating conditions, background radiation, and other random noise sources. When such noise due to environmental factors is present, its amplification can produce a total signal that is dominated by noise contributions rather than by the true signal that one desires to measure (the signal to be measured). To achieve maximum performance, it is highly desirable to remove as much environmentally generated noise as possible before performing the main signal amplification. Differential amplification techniques are helpful for achieving this. In general, differential amplifiers are used to detect small signals in the presence of interfering waveforms and random noise through the process of common mode noise rejection. Whether the interfering electromagnetic waves are periodic or random, the waves can appear simultaneously (within a time dictated by the speed of light and the physical separation of the signal and reference detector elements) at the detector elements. Typically, the charge resulting from an environmental source such as, for example, electromagnetic interference (EMI) is integrated using two independent preamplifiers connected to signal and reference detectors and the amplified signal and reference outputs are applied at the two inputs of a difference amplifier. This leads to amplification of noise as well as of any true signal that may be present.
Detection schemes for detection of ions, electrons, and other charged particles are especially susceptible to such environmental noise problems. For example, ion detectors are required in analytical instrumentation of the types known as ion mobility spectrometers, ion mobility-based explosive and chemical-warfare-agent detectors, and mass spectrometers. Ion mobility-based devices separate ions in a gaseous medium at pressures from approximately 1 Torr to atmospheric pressure. Mass spectrometers have historically required vacuum systems and operate at pressures lower than 10−5 Torr, although some recent designs operate at substantially higher pressures.
Mass spectrometer detectors are typically one of three types: 1) a single-point ion-neutralization electrode, often of the Faraday-cup type, followed by a current-to-voltage converter (IVC); 2) a single-point ion-to-electron converter followed by an electron multiplier; and 3) an ion-neutralization electrode followed by an integrating charge-to-voltage converter (CVC), which can be assembled into multipoint arrays.
The first two suffer from some inherent limitations. The IVC type suffers from relatively poor ion sensitivity. Intrinsic noise levels are on the order of 6000 ionic charges/s in state-of-the art devices. The electron-multiplier-based detector is susceptible to contamination in poorly evacuated mass spectrometers. Ions must arrive with sufficient kinetic energy to eject an electron from the surface. These ejected electrons are accelerated by a high voltage into the electron multiplier. Both contamination and poor vacuum lead to progressively higher spontaneous noise and lower sensitivity. Electron-multiplier-based detectors will not function at pressures about approximately 10−4 Torr because the ejected electrons either are slowed by collisions with background gas or react with the background gas.
The CVC-based detector can operate at all pressure and vacuum levels, is relatively immune to contamination of the ion receptor surface, and does not require the ions to arrive at the neutralization electrode with large kinetic energies.
While both IVC-based and CVC-based detectors have been used in ion-mobility-based sensors at pressures up to atmospheric pressure, both types require extensive shielding against electromagnet interference (EMI)-induced noise. What is needed for each is a means to reduce EMI sensitivity and sensitivity to other environmental noise sources while maintaining high sensitivity to analyte ions.
Capacitive transimpedance amplifiers (CTIAs) are suitable for use with CVC-type ion detectors. However, in a standard CTIA, the inputs are very sensitive to induced charge, so they are very susceptible to external electromagnetic wave interference (EMI) and other environmental factors. The most commonly encountered EMI is 60 Hz AC line noise and its second harmonic at 120 Hz. Although the detection limits of ion detectors built using standard CTIA technology are relatively low (comparable to electron multipliers under favorable conditions), even lower detection limits could be obtained if the influence of EMI and other environmental factors could be reduced. Resistance to EMI may be particularly important if such devices are to be used in particularly noisy environments. The differential capacitive transimpedance amplifier approach of this invention addresses this need and provides significant reductions in noise due to EMI and other environmental noise sources.
Differential amplifiers are useful for detecting small signals in the presence of interfering waveforms and random noise though the process of common mode rejection of noise. A differential amplifier or difference amplifier is constructed to produce a signal proportional to the difference in signal between the two inputs and is not sensitive to the absolute difference between the input signals and ground. This absolute difference is common to both inputs and is rejected by the differential amplifier, hence the term “common mode rejection.”
A type of CTIA currently used for ion detection in some ion mobility spectrometers is based upon a single-output differential amplifier circuit. It typically consists of a detector such as a Faraday cup connected to the inverting input of an operational amplifier (op amp). A capacitor is connected between the inverting input and the output of the op amp. A charge builds up on the input side of the capacitor that is proportional to the charge received by the Faraday cup. Charge that is induced by environmental factors on the input side also contributes to the charge that builds up on the capacitor, producing an undesirable noise contribution to the measured charge.
Gresham et al. (U.S. Pat. No. 6,809,313) describes a plurality of capacitive transimpedance amplifiers (CTIAs) adjacent to a collecting surface that are electrically connected to a plurality of conductive elements so that charge from an ion striking an element is transferred to the capacitor of the connected CTIA. A controller counts the charge on the capacitors over a period of time.